In Darwin's original formulation of his theory of
evolution, he emphasized the importance of the local environment in shaping how
organisms change through time. Over the past 2 decades, however, his assumption
that natural selection, as it is known, is invariably the driving force of
evolution has fallen somewhat out of favor. Some evolutionary theorists have
argued that "genetic drift," random gene changes that accumulate over
time, underlies the evolution of new species. Thus, even with natural
selection, evolution's course should be rather unpredictable and not likely to
be repeated time and time again, they concluded. But results reported in this
issue by two independent teams indicate that natural selection seems to be as
important as Darwin had thought, often overriding the
randomness of genetic drift.

Both teams took advantage of
nature's own evolutionary laboratory. Raymond Huey of the University of Washington, Seattle, and his colleagues studied a European
fruit fly, Drosophila subobscura, that was introduced into California some 20 years ago. As the researchers
report on page 308, they found that over the south-to-north range of the flies,
the insects have evolved larger wings, a change that parallels what happened to
this species in Europe.

Dolph Schluter of the University of
British Columbia (UBC) in Vancouver and his colleagues studied a very different
species, a stickleback fish living in three isolated lakes on British
Columbia's Pacific coast. In work described on page 306, the researchers report
that the same two species have formed in all three lakes. Each lake contains
one with hefty, bottom-dwelling individuals and one with streamlined
individuals that feed in the open water. Both studies provide strong evidence
confirming "the importance and strength of natural selection as the major
agent of evolutionary change," says Douglas Futuyma, an evolutionary
biologist at the State University of New York, Stony Brook.

Even the entomologists who first
noticed the distinctively black European fruit flies in California almost 20
years ago thought this species provided an opportunity to see evolution in
action. But Huey and George Gilchrist, now an evolutionary biologist at
Clarkson University in Potsdam, New York, and their colleagues were the first to
test whether the flies evolved the same way in the New World as they had in the
old. In 1997, they collected D. subobscura flies from 11 spots ranging from
just north of Santa Barbara, California, to north of Vancouver. The following
year, Huey and Spanish colleagues trapped the flies over roughly the same range
of latitudes in Europe, traversing the continent from southern Spain to the
middle of Denmark.

The team then raised the different
populations of flies, providing the same food and living conditions for them
all. After allowing a half-dozen generations to go by, the researchers measured
the wing lengths--an indicator of overall body size--of flies from each locale.
The results were striking, particularly in the females, says Gilchrist.

He and his colleagues saw an
increase in wing size--to a 0.1-millimeter difference, or 4%--in the European
flies collected from south to north. And they saw the same increase in the
fruit flies from North America, even though the species had spent only a brief
time on the continent. Indeed, Andrew Hendry of the University of
Massachusetts, Amherst, who has recently completed a survey of evolutionary
rates, says that the change "is as fast as I have ever seen. I think this
will shake up a lot of people." The adaptive significance of the change is
unclear. Still, says evolutionary biologist Jeff Mitton of the University of
Colorado, Boulder, the fact that it occurred twice in similar environments
makes for "a very clean and compelling story" in favor of natural selection.

The genetic basis of the change may
be different in the European and North American versions of D. subobscura,
however. Huey and his colleagues found that the European populations lengthened
the part of the wing closest to the body, while those in North America extended
the outer segment. The work shows that "there can be different ways of
attaining the same outcome," notes Futuyma, and thus some aspects of
evolution may still be random and unpredictable.

Schluter's team found that the
sticklebacks they studied represent an even more dramatic case of parallel
evolution. Originally of marine origin, the fish were trapped in coastal lakes
formed some 10,000 years ago by a retreating glacier. The lakes are isolated
from one another--indeed, two are located on separate islands along the
coast--yet each of the three lakes wound up with the same two noninterbreeding
varieties of stickleback, the bulky benthic type and the actively swimming
limnetic type.

To understand the basis of the
reproductive isolation, UBC's Laura Nagel, Janette Boughman, and Howard Rundle
tested the mating preferences of the fish. They found that females choose males
that look like themselves. For example, benthics mated with benthics, both from
their own lake and the others, while shunning all limnetics. "Whatever it
is that makes the benthics dislike the limnetics, it's happened over and over
again," Schluter explains. That finding, adds Mitton, was "a real
surprise" and shows that natural selection can yield new species.

The more researchers probe the
corners of nature's laboratory, the more evidence they are likely to find
supporting the importance of natural selection, Mitton says. For example, he
sees repeated patterns of evolution in some traits of the pinyon pines that he
studies. These examples "say that natural selection can cause a population
to change very quickly and hint that speciation could [occur] very
quickly," he notes. And that makes him even more sure that Darwin was
right after all.